This invention relates to metallic contacts for semiconductor devices, and more specifically relates to a novel semiconductor contact and process for its manufacture.
Metal contacts must be made to the surface of silicon wafers or chips which may have junction patterns therein which define particular kinds of semiconductor devices. The metallizing process should be capable of making ohmic contact to silicon regardless of the conductivity type of the silicon and should have low lateral impedance. Moreover, the metallizing should survive temperatures which might be encountered in subsequent manufacturing steps of the semiconductor device manufacturing process. The contact system should also be resistant to chemical etching process which may be subsequently used to complete the manufacture of a particular device and should be resistant to thermal fatigue which might be experienced during the operation of the device. Finally, the contact system should be easily solderable to electrical leads or other contact coatings.
Metallizing systems which are presently known do not satisfy all of the above characteristics. For example, nickel-chromium-nickel-silver systems are known where the silver is easily solderable top metal. These systems frequently tend to bubble off the silicon substrate and delaminate at the nickel-solver interface. Moreover, such metallizing does not survive subsequent process steps which may raise the temperature of the substrate to about 650.degree. C. Another commonly used contact metal is aluminum which bonds well to a silicon surface. However, if devices employing an aluminum contact are later subjected to an alloying operation or some other high temperature operation, the aluminum will diffuse into the surface to form P type silicon. Thus, the contact may not be useful in devices in which the surface beneath the contact should remain of the N-type.